The turbine section of a gas turbine engine contains a rotor shaft and one or more turbine stages, each having a turbine disk (or rotor) mounted or otherwise carried by the shaft and turbine blades mounted to and radially extending from the periphery of the disk. A turbine assembly typically generates rotating shaft power by expanding hot compressed gas produced by combustion of a fuel. Gas turbine buckets or blades generally have an airfoil shape designed to convert the thermal and kinetic energy of the flow path gases into mechanical rotation of the rotor.
Turbine performance and efficiency may be enhanced by reducing the space between the tip of the rotating blade and the stationary shroud to limit the flow of air over or around the top of the blade that would otherwise bypass the blade. For example, a blade may be configured so that its tip fits close to the shroud during engine operation. Thus, generating and maintaining an efficient tip clearance is particularly desired for efficiency purposes.
Although turbine blades may be made of a number of superalloys (e.g., nickel-based superalloys), ceramic matrix composites (CMCs)) are an attractive alternative to nickel-based superalloys for turbine applications because of their high temperature capability and light weight. However, CMC components must be protected with an environmental barrier coating (EBC) in turbine engine environments to avoid severe oxidation and recession in the presence of high temperature steam.
Thus, in certain components, regions of the EBC may be susceptible to wear due to rub events with adjacent components. For example, for the CMC blade, the EBC at the blade tip is susceptible to rub against metal shroud components. If the EBC coating wears away, the CMC blade is then open to recessive attack from high temperature steam that will open up the clearance between the CMC blade tip and the metal shroud, thereby reducing the efficiency of the engine.
Thus, it is desirable in the art to provide materials and methods for reducing EBC wear on a CMC blade tip caused by a rub event during operation of a turbine.